Hopi Point, Grand Canyon, Circa 1955

In this photo gallery, get a "then and now" look at three iconic national parks—Grand Canyon, Yellowstone, and Biscayne—and see what changes time has wrought.

Here, visitors gather at Hopi Point, a promontory that affords views of the Grand Canyon 45 miles (72 kilometers) eastward and westward. At the time of the photograph, the Colorado River still made its natural run through the canyon. In the 1960s the Glen Canyon Dam diverted the river’s flows for power production and the growing West’s thirsty communities.

Mather Point, Grand Canyon, 2006

Photograph by John Moore, Getty Images

Tourists take in the view and pause for photographs at Mather Point on the Grand Canyon’s popular South Rim. Today the canyon is threatened by lack of natural water flow, uranium mining, and tourist overflights that number around 45,000 a year, changing the experience for visitors inside the park.

Old Faithful, Yellowstone, 1883

Photograph by Jay F. Haynes, Library of Congress

Visitors to Yellowstone National Park watch the eruption of Old Faithful, the most famous of the park’s geysers, in 1883. Established in 1872 in Wyoming, Idaho, and Montana, Yellowstone is America’s oldest national park.

Old Faithful, Yellowstone National Park

Photograph by Robin Smith, Photo Library

Crowds gather to watch the eruption of Yellowstone’s Old Faithful. Home to half the planet’s hydrothermal features, the park is a geothermal hot spot that periodically draws attention as a potential source of alternative energy. Other issues facing the park today include invasive species and snowmobiles, one of many usage conflicts caused by the park’s popularity.

Biscayne National Park, Circa 1991

Photograph by Stephen Frink, Corbis

Established in 1980, Biscayne is the largest marine park in the National Park System. Located just five miles (eight kilometers) from the heart of downtown Miami, the underwater wilderness and mangrove shoreline first came under threat from developers in the 1960s. Conservationists campaigned to protect it, and it became a national monument in 1968.

Biscayne National Park

Photograph by Michael Melford, National Geographic

Recreational boats pull up to Boca Chita, an island in Florida’s Biscayne National Park. Ninety-five percent of Biscayne’s 173,000 acres (70,000 hectares) are covered by water, and most of its half million annual visitors arrive by boat. Many do so without ever realizing they’ve entered a national park.